Status of MEG Physics Analysis
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Transcript of Status of MEG Physics Analysis
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Fabrizio Cei 1
Status of MEG Physics Analysis
Fabrizio CeiINFN and University of Pisa - Italy
BVR PSI, 17 February 2010
17 February 2010
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Fabrizio Cei 2
Outline
Results of physics analysis of 2008 data.Present status of physics analysis:- summary of physics analysis for individual
sub-detectors;- observation of radiative decay events
in MEG 2009 data;Perspectives for 2009 data analysis.
17 February 2010
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Fabrizio Cei 3
2008 Data Analysis
17 February 2010
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Fabrizio Cei 417 February 2010
2008 run: 1014 m+ stopped in target
RD
RD
RD
RD
RDProgrammed
beam shutdowns
RDCooling system
repair
Air test in COBRA
We also took RMD data once/week at reduced beam intensity
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Fabrizio Cei 517 February 2010
Pre-selection/blinding windows
Open files: 16 % events
Pre-selection process repeated several times with improving calibrations and algorithms. Final blinding box: 1 ns around zero (timing offset subtracted).
Plane (Eg, Dt) used for pre-selection + reconstructed track with associated TC hit
Blinded files: 0.2 % events
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Fabrizio Cei 617 February 2010
fS/fM is obtained by generating and reconstructing large samples of MC events (for signal and Michel) in various configurations to take into account the DCH instabilities
TRG = 22: Michel events trigger (only DCH track required)TRG = 0: MEG events trigger
Normalization 1)SESeBRkeBRN e /)()( gmgmg
++++
)( )( )|()|()|(
M
S 2222
0 PsctrackATCetrackTRG
eTRGffNk
me
+
+
gg
g
MM
SeeS
fpTCDCptrackDCAf
)MeV|( )|MeV,( )( 5050
where:
107 pre-scaling factor
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Fabrizio Cei 717 February 2010
Normalization 2)Final Value
k = 4.7∙1011 ± 10%
Independent computation k = 4.9∙1011 ± 10%
Advantage of this technique: it uses the MEASURED number of Michel positrons instead of the CALCULATED number of stopping muons/second it is independent of time varying DCH acceptance and efficiency. Result confirmed by computations based on time-averaged acceptance and efficiency.
k = 1/”SES” (not exactly a “SES”: not zero bck)
2
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Fabrizio Cei 817 February 2010
Generalities on analysis
Three independent blind-likelihood analyses. RD and accidental event rates in the signal region fitted or
estimated a priori by means of side-bands information. Feldman-Cousins method for C.L. determination.Kinematical variables used: - Positron and Gamma Energies; - Relative timing and relative angle; Likelihood function:
++
obsN
i
BGRDSig
obs
obsN
BGRDSig BNNR
NNS
NN
NNNNNNL
1!exp,,
Nobs = number of observed events
Signal PDFRD PDF
Accidental BCK PDF
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Fabrizio Cei 917 February 2010
Analysis: PDF determination 1)
Signal: Gamma Energy from p0 (DRS) or MC taking into account resolution (TRG); Positron Energy: 3 gaussian shapes; sigma’s extracted from Michel positron fit; Positron-Photon relative angle: toy MC based on experimental angular resolutions of positrons and photons; Positron-Photon relative timing: gaussian shape with sigma = 147 (or 135) ps from radiative decay data fit with Gamma Energy outside of Blinding Box.
Radiative Decay: Gamma Energy + Positron Energy + Relative Angle e+-g: 3-D distribution based on theoretical shape folded with detector response (correlations); Relative Timing e+-g: gaussian shape withsigma = 147 (or 135) ps as for signal.
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Fabrizio Cei 1017 February 2010
Analysis: PDF determination 2)
Accidental bck: fit of Gamma Energy, Positron Energy and Relative Angle for events falling in DT “side bands” |DTeg| > 1 ns
Accidental radiative decay + Positron Annihilation in flight + resolution + pileup
Gamma Energy
Important point: the PDF of the most dangerous background can be measured !
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Fabrizio Cei 1117 February 2010
Sensitivity evaluationExpected sensitivity evaluated with two methods:
Toy MC assuming zero signal (two independent calculations):
- generated 1000 independent samples of events using bck and RD pdf’s; - upper bound on number of signal events evaluated for each sample;
- average upper bound @90% C.L: 6 events - average upper bound on B.R.(m → eg) = 1.3 x 10-11.
Fit to events in the sidebands:
- applied same fitting procedure used for data in the signal region; - upper bound: B.R.(m → eg) (0.9 2.1) x 10-11.
Comparison: present upper bound from MEGA experiment: 1.2 x 10-11
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Fabrizio Cei 1217 February 2010
Likelihood analysis
7.140 SigN
Results in close agreement from the three analyses.
Checks: Number of RD events in
agreement with predictions and extrapolations from sidebands;
Test of C.L. extraction with bayesian technique;
Fits using TRG instead of DRS information and different DT computation algorithms;
Fit to DT only (sensitive to signal + RD).
Fit in the signal region
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Fabrizio Cei 1317 February 2010
Eg vs Ee+
Cut at 90% efficiency on other variables (relative angle and relative timing)
103 MC events
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Fabrizio Cei 1417 February 2010
B.R.(m → eg) 90 % C.L. limit
From the 90 % C.L. upper bound on number of signal events:
NSig 14.7 we obtained the corresponding 90 % C.L. upper limit:
BR(m+ → e+g) 2.8 x 10-11
2 times worse than the expected sensitivity. The probability of getting this result by a statistical fluctuation of the observed distributions is (3 5) % (Bad Luck !)
Results available at arXiv:0908.2594.Paper to be submitted soon.
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Fabrizio Cei 15
Present status of physics analysis
17 February 2010
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Fabrizio Cei 16
DCHSeveral quantities to be calibrated:
Common timing offset (t0);Relative timing between end of wires ( r);Anode and pads charges ( z).
Standard calibrations completed and implemented.
Alternative calibrations/algorithms under way to:Check the origin of not yet adequate
performances by means of independent codes;Try to improve such performances.
Alignment check by cosmic rays measurements.17 February 2010
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Fabrizio Cei 17
DCH standard calibrations: z anodes
17 February 2010
Size of pad pitch
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Fabrizio Cei 18
DCH standard calibrations: z pads
17 February 2010
Vertical offset Cathode
Hood
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Fabrizio Cei 19
Relative timing between ends of wires
17 February 2010
Ch’s with sync problems
(Time_end0 – Time_end1) vs Wire
Shows relative offsets before and after calibration. The RMS of the not calibrated data seems to be dominated by deviant wires.
The relative offsets of the calibrated data show a double peak. Since, this double peak structure comes about after calibration, it’s good indication that there is a pattern in the leading edge fits, or, first peak remained while deviant data points populated second peak. Under investigation.
Low Track Statistics
(T_end0 –T_end1) vs wire
Low GainChannels with sync Offsets
calibrated
not calibrated
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Fabrizio Cei 20
Alternative algorithms/calibrations
17 February 2010
Z extrapolated from track
Integration based on trapezoidal method
Fit each separated charge independently as a function of z
T (ms)
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Fabrizio Cei 21
Single hit Z resolution
17 February 2010
score = 480 mm, core fraction = 0.722, mean -35 mm (asymmetric tail)
cm
Double gaussian fit |DZ| > 0.3 cm
|DZ| > 0.3 cm
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Comparison of performances
17 February 2010 Fabrizio Cei 22
All these numbers were obtained by using the double-turn method they must be divided by sqrt(2) sf 9 ÷ 11 mradsq 15.5 ÷ 17 mradsP 0.51 ÷ 0.61 MeV
Numbers in parentheses () are RMS of distributions.
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Fabrizio Cei 23
Preliminary analysis of cosmic ray data
17 February 2010
Tracks fitted by 3-D straight lines. Computed R and Z distance of track from nominal position of each chamber.
Mean R -440 micronMean Z -190 micron
ChamberChamber
Chamber Chamber
IMPROVED, Starting point for new iteration.
Analysis in progress
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Fabrizio Cei 24
DCH: coherent noise
17 February 2010
Low frequency ( 13 MHz) noise; more visible on pads.
-0.0000010 -0.0000005 0.0000000-6
-4
-2
0
2
4
6
8
10
Am
plitu
de, m
V
Time, s
dc00A cell 4 end 6 Double sin fit subtracted
rms = 1.2 mV
Can be fitted with a single or double sinusoidal shape and subtracted
• The remaining noise is more “white” then before• RMS reduced from 2.7 mV to 1.2 mV – more then twice• This is just an example, dominated frequency are visible in almost all WFs• Possible tool to improve Z resolution
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Fabrizio Cei 25
DCH: MC studies
17 February 2010
Angular resolution depends on Z resolution, but not dramatically; To obtain a 15 mrad angular resolution one should use an abnormally bad value sZ = 3 mm.
Usual way of measuring the angular resolution (double turn method) gives a systematically worse result than the difference wrt true value (2÷3 mrad shift).Resolution for signal events is better than for Michel events.
1: 300 micron, single gaussian2: 470 micron, single gaussian3: double gaussian
1
12 23
3
Event Type MC Z Sigma Angular Resolution q
Michel 100 mm 7.0 mradMichel 300 mm 7.2 mradMichel 470 mm 7.6 mrad
Michel 470 mm (70%) + 1.5 mm (30 %)
7.9 mrad
Signal 100 mm 6.0 mradSignal 300 mm 6.4 mrad
Signal 470 mm 6.6 mrad
Signal 470 mm (70%) + 1.5 mm (30 %)
7.0 mrad
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Fabrizio Cei 26
DCH: present situation
17 February 2010
sq = 12.7 mrad
sf = 8.1 mrad
sZ = 3.1 mm
sR = 2.4 mm
≥ 8 hits, TC+DCH match as in 200850 MeV < E < 54 MeV
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Fabrizio Cei 27
DCH: CommentsComparison between MC and present level of analysis (all in mrad):
Data (2-turn) MC (2-turn) MC (true) Michel q 12.7 10.5 8Signal q not avail. 8.6 7Michel f 8.1 8.0 7 fulfilled !Signal f not avail. 7.1 6.5
Central column is our goal ! Not too far, but several possible actions … - Refine single hit Z calibrations;- Check the hit reconstruction/pattern recognition/tracking algorithms;- Check the database;- Refine comparisons with MC (number of hits, inefficiencies, materials …);- Improve CR data analysis misalignment corrections, survey …. - Subtract coherent noise;- Understand and correct other effects which can deteriorate the resolution (cross talks, chamber breathing …)
17 February 2010
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Fabrizio Cei 28
Timing Counter Several calibrations:
- Time Walk with triple events intrinsic bar resolution;
- z-offset;- effective velocity (under way);- timing calibration with MEG (Michel) and Dalitz
events inter-bar and Teg (Boron for check); All constants in the database.
17 February 2010
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Fabrizio Cei 29
Timing Counter cnt.
17 February 2010
Intrinsic resolution is a bit worse than in 2008.
Studies under way to figure out the origin of the discrepancies. However, (70 ÷ 100) ps is still acceptable.
Bar instrinsic resolution
MEG Data.Z offset calibration with Michel events
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Fabrizio Cei 30
Timing Counter cnt.
17 February 2010
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Fabrizio Cei 31
Timing Counter: Comments
TC bars stable during 2009 data taking; TC resolutions look a bit worse than in 2008; We expect some improvements from the next re-
processing (better TC-DCH match; more accurate determination of calibration constants …);
Some calibrations are under refinement (effective light velocity, inter-bar timing …);
MC studies of Dalitz events to disentangle the contribution of track length uncertainty;
….
17 February 2010
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Fabrizio Cei 32
Xenon Calorimeter
17 February 2010
Calibration chain almost completed: LED/Gain variation with time; Q.E. determination by alpha data in liquid Xenon (in 2008: gaseous
Xenon); Uniformity corrections; Pedestal determination (CEX-m difference); Timing constants for two different algorithms; Full implementation in the database under completion; Everything ready before the next re-processing (sometimes in March).
Checks: Uniformity of CW line energy; Linearity.
Results: Energy resolution with p0; Background spectrum.
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Fabrizio Cei 33
Xenon Calorimeter cnt.
17 February 2010
10% decrease
Gain decrease correction
Effect of first QE set.Further refinementsunder way to precisely match the calorimeter optical properties:
- reflections- Rayleigh scattering …
3.3 %
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Fabrizio Cei 34
Xenon Calorimeter cnt.
17 February 2010
Uniformity of 17.6 MeV CW Li peak. Each point represents the reconstructed position of the Li peak in a 3-D spatial bin.
Linearity curve determined by using p0.CW points look displaced by 1% (under study).
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Fabrizio Cei 35
Xenon Calorimeter cnt.
17 February 2010
CEX energy resolution (FWHM) as a function of time. Purple line obtained by inserting pedestal fluctuations in MEG runs.
supper = 2.0% in a 2x2 PMTs grid(usual 1x1); 1.95% for collimator #8.
Background spectrum.Fit with a combination of RD+AIF (Green), and Pile-up (Blue). - Resolution just a bit worse than in 2008; - Energy scale not exactly 1 because of a mistake in configuration parameters (data to be reprocessed).
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Fabrizio Cei 36
Xenon Calorimeter: Comments
Calibration chain used in 2008 looks adequate. Work in progress for further improvements (i.e. Q.E determination); Energy resolution a bit worse than in 2008, but analysis work still preliminary. Nevertheless, 5% FWHM goal in energy resolution already fulfilled. Timing resolution requires further efforts:
- single PMT timing extraction; - boards calibration/synchronization; - ….
First background energy spectrum preliminary PDF.17 February 2010
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Fabrizio Cei 37
RMD observation 1)
17 February 2010
Single bars = 209 ps
All bars
Three independent analyses, with different cuts or no cuts at all. Peak well visible above background without need of refined searches.
Offset subtracted, but mean displaced by 300 ps
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Fabrizio Cei 38
RMD observation 2)
17 February 2010
Pre-selection window needs to be re-centered. Resolution is some tens of ps worse than at the end of 2008 (XEC/TIC calibration constants ? tracking ? To be addressed)
RMD signal stable along the data taking period.
In 2008 the signal became less and less visible because of the reduced DCH efficiency.
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Fabrizio Cei 39
Perspectives for 2009 data analysis
17 February 2010
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2009 Data Sample
Fabrizio Cei 40
preliminary
Short run, but very smoothImproved tracking and trigger efficiency 6.5 x 1013 muons stopped in the target
17 February 2010
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Fabrizio Cei 41
Perspectives for 2009 data 1)
With respect to 2008 the starting conditions of our analysis are generally better:
- Xenon calorimeter stable at predicted light yield;- stable behaviour of DCH; only few channels not working properly;- higher trigger efficiency 88% because of better direction
match LUT.
The analysis scheme we plan to use is similar to that of 2008, which allowed to produce our first paper:
- 2 ÷ 3 complete reprocessing, starting from waveforms (< 2 weeks per each);
- some ( 5) reprocessing of pre-selected data (few days per each);- pre-selection/selection/blinding windows to be refined.
17 February 2010
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Fabrizio Cei 42
Perspectives for 2009 data 2)
A strong calibration effort is under way on all sub-detectors, based on the first data processing.
- Calibration constants still to be optimized; - However, resolutions close to that obtained last year at the end of analysis chain;- DCH angular resolutions already better than in 2008;
continuous optimization work by a dedicated working group;- Possible improvements by alignment check;- Xenon calorimeter energy resolution at 5% FWHM level;- Improvements in timing resolution expected from DRS timing calibration;
Selections and cuts to be optimized to reach the best compromise between resolutions/background rejection and efficiency.
Algorithms for final analysis (i.e. likelihood) well tested on 2008 data.17 February 2010
A detailed job list with time schedule will be prepared as in 2008 !
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Fabrizio Cei 43
Physics analysis schedule for 2009
17 February 2010
Finalize Calibrations/resolutions
Final reprocessing
BG Estimation
Normalization
Systematics
Feb
Final analysis
Open box
Mar Apr May Jun Jul
ICHEP2010ICHEP2010(submission)
Reprocessing
Next reprocessing(s)
Various checks of final result
Update
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Fabrizio Cei 44
Performance Summary measured in sigma
2008 2009(preliminary) 2010(preliminary) “Goal”
Gamma Energy (%)Gamma Timing (psec)Gamma Position (mm)Gamma Efficiency (%)
e+ Timing (psec)e+ Momentum (%)
e+ Angle (mrad)e+ Efficiency (%)
e+-gamma timing (psec)Muon Decay Point (mm)
Trigger efficiency (%)
2.0(w>2cm)80
5(u,v)/6(w)63
<1251.6
10(φ)/18(θ)14148
3.2(R)/4.5(Z)66
←>67←←←
0.858(φ)/11(θ)
40<180
2.2(R)/3.1(Z)88
1.5(w>2cm)68←←900.7
8(φ)/8(θ)40120
1.4(R)/2.5(Z)94
1.243
3.8(u,v)/5.9(w)6050
0.3-0.38(100%)3.8-5.1
9064
0.9-1.1100
Stopping Muon Rate (sec-1)DAQ time/Real time (days)
3×107 (300μm)48/78
2.9×107(300μm) 35/43
3×107 (300μm)133/162
3×107
300/-
S.E.S @90% boxExpected NBG
SensitivityBR upper limit (obtained)
5×10-12
0.51.3×10-11
2.8×10-11
2.3×10-12
0.76.6×10-12
-
4.0×10-13
0.9 1.3×10-12
-
3.8×10-14
0.51.0×10-13
-
17 February 2010
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Fabrizio Cei 45
2009 Expected Sensitivity
Estimate by means of toy MC simulation/Feldman-Cousins.– Updated resolutions and data statistics in table– NBG in analysis window estimated by scaling NBG in analysis window in
2008– A little narrower analysis window (signal efficiency x 0.95)– NBG expected in analysis window: 435
NBG in (old) 90% signal box: 1.1
Results:– Average Nsignal upper limit: 6.9– Average BR upper limit: 6.6×10-12
• 6.9 × (2.0 × 10-12)/2.2/0.9517 February 2010
2008 sensitivity in analysis window
Improvement in data statistics wrt 2008
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Fabrizio Cei 46
Expected Sensitivity for 2010 or later
Estimate by means of Poisson statistics/Feldman-Cousins – Updated resolutions and data statistics in table– NBG in signal box is estimated by the PDF probability ratio.– Asymmetric 90%-efficiency signal box to optimize S/N (same efficiency but less BG)
Results– 2010 (stat. ratio to 2008: 11.4)
• S.E.S.: 4.0 × 10-13
• NBG: 0.9• UL: (4.0 × 10-13 ) × 3.2 = 1.3 × 10-12
– 2010 + 2011 + 2012 (stat. ratio to 2008: 11.4 + 13.4 + 13.4)
• S.E.S.: 1.2 × 10-13
• NBG: 3.0• UL: (1.2 × 10-13 ) × 4.41 = 5.3 × 10-13
17 February 2010
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Fabrizio Cei 47
Backup slides
17 February 2010
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Fabrizio Cei 4817 February 2010
Analysis cuts Track quality cuts (minimum number of hits and chambers, good chi2 for fit …); Selection of track with best pattern recognition; Track timing determined by position matching between DCH and TC; Elliptical cuts on target and beam spot at COBRA centre.
(Already used to evaluate normalization factor by means of Michel positron events)
Cosmic ray rejection based on front/back charge ratio on the LXe calorimeter; Fiducial volume cut for photons; Pile-up identification in the LXe calorimeter and photon energy correction; Collinearity cut on photon-positron relative angle; 50 MeV < Ee < 56 MeV; 46 MeV < Eg < 60 MeV; |Dteg| < 1 ns;
Multiple algorithms and two independent digitizers for photon energy/timing checks !
Positron
Photon and photon-positron correlation
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3-D view of a MEG event
Positron Track
Hits on DCH
Hits on TC
Photon Trajectory
Hits in XEC
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Fabrizio Cei 5017 February 2010
Events in signal region vs pdf’s
Distributions normalized to the total number of events Black: real events Red: signal pdf Blue: RD pdf Green: accidental pdf
Positron Energy Gamma Energy
Gamma Positron Angle
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Systematic Effects 1)
% %
psecpsec
NSig(90 % C.L.)
NSig(90 % C.L.)
Analysis repeated by changing one parameter at a time and building new pdfs.
0.6 % error on Eg scale DNSig= 0.6
Eg s(Eg
s(Dt Dt
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Fabrizio Cei 5217 February 2010
Systematic Effects 2) sf, sq = (10 mrad, 18 mrad) x X
Assuming 10% error DNSig = 0.35
Positron angular resolution
Positron Energy Scale
300 keV error DNSig 1
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Timing Counter cnt.
17 February 2010
Teg distribution obtained with Dalitz sample.
DT (ns) DT (ns)
Blue: 2009Red: 2008 (final reprocessing)
Check with Boron sample
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Fabrizio Cei 54
Xenon Calorimeter cnt.
Timing resolution
Intrinsic timing resolution is good ( 45 ps); Electronic + analysis contribution to LXe-TC timing resolution measured by using a split pulse. Sigma (105 ÷ 120) ps, dominated by inter-board synchronization. Lower contributions from smoothing functions (different for DRS2 and DRS4) and same board synchronization.
17 February 2010
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Comments Gamma energy 1.5% (2010): resolution measured at the best position in 2009Gamma timing 68 ps (2010): resolution measured at CEX in Dec.2008. Better than resolution quoted for run2008 (80ps) due to light yield improvement.Positron timing <125 ps (2008/2009): 148(e-γ)⊖80(Xe), including DRS effect partially.Positron timing 90 ps (2010): 70(TC) ⊕ 60ps (path length, MC)Positron-Gamma timing 120 ps (2010): 70ps (TC) ⊕ 60ps (path length, MC) ⊕ 68ps (Xe). No contribution from DRS.Positron momentum 1.6% (2008): average over core and 2 tailsPositron momentum 0.85% (2009): single GaussianPositron momentum 0.7% (2010): single Gaussian
Positron angle 8 mrad(f), 11 mrad(θ) (2009): resolution measured in MC by two-turns track method Positron angle 8mrad(f), 8mrad(θ) (2010): resolution measured in MCPositron efficiency 40% (2009/2010): measured in 2009 with trg22Muon decay point 2.2mm(R), 3.1mm(Z) (2009): resolution measured by two-turns track method Muon decay point 1.4mm(R), 2.5mm(Z) (2010): resolution measured in MC by two-turns track method DAQ time: (real) days for the MEG trigger DAQ time (Peter’s estimation)NBG in 90% signal box (2008/2009): recalculated with NEW asymmetric signal box.
17 February 2010